Wife's 48

[Tempest67]

I think you should start adding metal before you cut out any more.

I see the 2 inner braces on the new firewall (interior). I think I would start by attaching them to the upper firewall with sheet metal screws. This will allow the new firewall to hang in place while you position the floor pieces, and allow you to move it up or down as needed. (reposition the sheet metal screws)

I see your floor pieces include about 12 inches of firewall. My 51 has a seam at the floor/firewall that bends outward, and that is where the 2 pieces are spot welded together.... is that what you want? or you could cut a straight edge on the floor piece, just above the bend, and use that to lap weld the pieces together?...not factory, but plenty strong.

many ways to 'skin this cat', just take your time, and don't start welding till everything fits together........... just my 2 cents

Keep us posted, and enjoy the process.